1. Field of the Invention
The present invention relates to the field of edging optical articles, such as ophthalmic lenses, more particularly those comprising, as an outermost layer, an anti-fouling top coat such as a hydrophobic and/or oleophobic surface coating.
An ophthalmic lens results from a succession of molding and/or surfacing/smoothing operations determining the geometry of both convex and concave optical surfaces of said lens, followed by appropriate surface treatments.
The last finishing step of an ophthalmic lens is the edging operation consisting in machining the edge or the periphery of the glass so as to shape it according to the required dimensions to fit the lens into the eyeglass frame wherein it is to be arranged.
Edging is conventionally performed on a grinding machine comprising diamond abrasive wheels that perform the machining step as defined hereinabove.
The lens is held, upon such an operation, by axially acting blocking members.
The relative motion of the lens with respect to the grinding wheel is monitored, generally digitally, so as to provide the desired shape.
As it appears, it is absolutely imperative that the lens be firmly maintained upon such a movement. To this end, before the edging operation, a lens-blocking step is performed, i.e. a holding means or chuck is positioned on the convex surface of the lens.
A holding pad, preferably a two-sided adhesive, is arranged between the chuck and the convex surface of the lens.
The so-equipped lens is positioned on one of the above-mentioned axial blocking members, the second axial blocking member clamping then the lens on the concave surface thereof by means of an abutment, generally made of an elastomer.
Upon the machining step, a tangential torque stress is generated on the lens, which may result in a rotation of the lens relative to the chuck if the lens holding means is not sufficiently efficient.
The good holding of the lens mainly depends on the good adhesion at the interface of holding pad/convex surface of the lens.
2. Description of Related Art
It is a common practice in the art to coat at least one main surface of a lens substrate, such as an ophthalmic lens or lens blank, with several coatings for imparting to the finished lens additional or improved optical or mechanical properties. These coatings are designated in general as functional coatings.
Thus, it is an usual practice to coat at least one main surface of a lens substrate, typically made of an organic glass material, with successively, starting from the surface of the lens substrate, an impact-resistant coating (impact resistant primer), an abrasion- and/or scratch-resistant coating (hard coat) and an anti-reflection coating.
The last generation ophthalmic lenses most often also comprise an external layer of anti-fouling material deposited on the anti-reflection coating, in particular an anti-reflection coating made of an inorganic material, so as to reduce their strong tendency to staining, for example towards fatty deposits. Such an anti-fouling top coat is generally a hydrophobic and/or oleophobic coating, which reduces the surface energy so as to avoid the adhesion of fatty stains, which are thus easier to remove. The hydrophobic top coat constitutes, in the finished optical article, the outermost coating.
Such top coats are well known in the art and are usually made of fluorosilanes or fluorosilazanes i.e., silicones or silazanes bearing fluorine-containing groups. Examples of classical materials for top coats are OPTOOL DSX, which is a fluorine-based resin comprising perfluoropropylene moieties, commercialized by Daikin Industries, KY130 from Shin-Etsu Chemical and KP 801M, also commercialized by Shin-Etsu Chemical. These coatings impart to the lens a contact angle with water of at least 100°.
This type of surface coating may be of such efficiency that the surface energy is considerably decreased. As a consequence, the adhesion at the pad/convex surface interface can be thereby altered, making difficult satisfactory edging operations, in particular for polycarbonate lenses, the edging of which generates much more important stresses in comparison with other materials.
High slipperiness of the lens surface may result in substantially shifting the center of the lens from the lens chuck in which the center of the lens is fixed, thus shifting the optical and horizontal/vertical axes of the lens from the real center thereof. This phenomenon possibly leads to depreciation of vision correcting effects and/or lowering of the visual power of the user wearing the eyeglass, and/or impossibility to insert the lens into the desired frame. The consequence of a badly performed edging operation is thus the pure and simple loss of the lens.
To solve these problems, it has been found advantageous to deposit on the outermost layer of the lens a temporary film.
An effective solution is to deposit on the outermost layer of the lens a temporary coating imparting a surface energy of at least 15 mJ/m2, in particular a MgF2 temporary layer, such as disclosed by the applicant in French Patent No 2824821. This solution is applicable whatever the deposition process of the anti-fouling top coat and especially if the anti-fouling top coat is deposited through vacuum evaporation. However, if the anti-fouling top coat is deposited by dip coating, the lenses may be more difficult to edge, particularly if said top coat is OPTOOL DSX from Daikin Industries. Moreover, lenses having their external layer coated with a temporary MgF2 layer may sometimes not be edged for 48 hours following deposition of such a temporary layer.
Such a method was further improved in French Patent No 2856056 in the name of the present applicant, which describes ophthalmic lenses coated with said MgF2 temporary layer, on which a layer of at least one non fluorinated metallic oxide and/or at least one non fluorinated metallic hydroxide has been deposited or created. The additional layers enable to perform the edging operation very quickly after the various layers have been deposited on the lens. However, the deposition or creation of such layers lengthens the process.
Japanese Patent Application published under No JP2004-122238 discloses a method to prevent slippage of a lens coated with an anti-fouling top coat during edging operation. The method comprises in a first step deposition onto the hydrophobic and/or oleophobic top coat of a first layer made of a resin material such as poly(vinyl acetate) and then in a second step sticking on said first layer an adhesive film made of a material such as polyethylene, polyethylene terephthalate (PET) or polypropylene. Adhesion of the first layer to the film is promoted by a silicone binder. The lens can then be fixed to the support of the edging machine by using a two-sided adhesive tape.
International Patent Application WO 2005/015270 to S. W. Nam describes formation on the surface of a lens of a temporary film exhibiting a greater coefficient of friction than the lens, in order to prevent a shift of axes and surface damage when processing the lens. The film is formed from a chlorinated polyolefin resin or PET, preferably a chlorinated polypropylene resin and is generally deposited onto a fluorinated anti-fouling material or a water repellent layer.